Two different approaches are known from the prior art which attempt to solve the problems of coordinating several images from different imaging positions.
Generally a tooth is optically imaged from different imaging positions, as it is only possible to image one certain area of the tooth on account of the limited imaging area of an imaging system. The obtained images, which jointly cover the entire tooth, are brought together on completion of the imaging to give a single 3D-data record which clearly describes the contour of the tooth. The prior art presents various methods for bringing the images together and calculating the 3D-data record, which methods will be discussed briefly in the following.
Basically, the coordinate data of the imaged tooth contour determined in one image and relating in each case to a system of coordinates which is dependent upon the position of the imaging system must be transformed into a common system of reference coordinates. Such a transformation is rendered possible, on the one hand, in that before the imaging small calibration bodies are arranged around the tooth. The individual imaged areas of the tooth must be selected so that at least two identical calibration bodies are visible on adjacent images. As the external dimensions of the calibration bodies and their exact position in space are measured with great accuracy at the beginning of the imaging sequence; adjacent images and the determined relative coordinate data of the tooth contour can thus be correlated with each other in a defined manner in each case by means of a transformation matrix which indicates the change in location (W090/05483).
The disadvantage of this method is that the use of calibration bodies is unpleasant for the patient and proves to be a complicated procedure for the dentist. Moreover, these calibration bodies, in so far as they are fitted on the tooth, cover a portion of the tooth which is thus not visible in the image. For this reason, the selected calibration bodies ought to be as small as possible, in which case, however, exact measurement is impaired, leading to inaccurately calculated coordinate data.
A further method for optical 3D-measurement of teeth or groups of teeth in the oral cavity of a patient consists of determining absolute coordinate data of the tooth contour with the aid of a rigid multiple arrangement of projection and imaging optical systems in a mouth probe head. The use of calibration bodies can be foregone, as the individual positions in space relative to a common system of coordinates are known on account of the rigid arrangement of the projection and imaging systems. The disadvantage of this method, however, lies in the fact that a large number of imaging and projection channels are required, which channels have to be guided via endoscopic systems or light guides from the oral cavity to the relevant image sensors and projection units. As the portion of the imaging and projection apparatus introduced into the oral cavity is to be separable for the purposes of disinfection and cleaning, the large number of optical endoscopic channels signifies a considerable expenditure on account of the necessary high mechanical precision of the coupling mechanism.